This invention relates to surge arresters (also known as lightning arresters and surge diverters) and more particularly to surge arresters of the multi-gap type that are used extensively in electrical systems of moderate voltages, say from 2.4 kv to 34.5 kv and sometimes on up to 100 kv. Arresters of this type are used in large numbers to protect transformers and other items of equipment in electric power distribution systems. They function to divert voltage surges of more than a predetermined value harmlessly to ground, thus protecting the equipment with which they are associated from excessive voltages, whether the voltage is derived from a lightning stroke or from another surge such as a switching surge or the like. The arresters must function very promptly to sparkover at the desired voltage and then, when the surge has been dissipated, must promptly shut off the power follow current that would otherwise continue to flow to ground. After the power follow current has been shut off the arresters must again be ready to resume their protective function of diverting surges to ground.
In general such arresters comprise a series of elements providing a plurality of spark gaps disposed within a porcelain housing which also contains a non-linear resistance usually made up of a plurality of non-linear resistors commonly called "valve blocks." The valve blocks are interposed in the circuit between a line to be protected and ground in series with the spark gap units. Sufficient gap units of proper dimensions are employed to produce the desired sparkover voltage. Upon sparkover, the voltage drop across the gaps is reduced very promptly and under the high voltage and current conditions of a lightning stroke or other surge the resistance of the valve blocks is also low so that the total voltage drop across the arrester is below the protective level required for the apparatus with which the arrester is associated. In simple multi-gap arresters of this type the power follow current is ordinarily shut off at the first current zero after the passage of the surge that caused the arrester to become conductive. The construction must be such that the arc will not restrike in the arrester under normal power system voltage even though the housing of the arrester may contain ionized gas.
Multi-gap arresters are known that are made up of a series of metal plate electrodes spaced apart by insulating studs or spacers to provide arc gaps. Arresters of this type are useful for moderate voltage services. The stack of electrodes making up the gap units and the valve block or blocks are enclosed in a porcelain housing, one end of which is connected to a conductor of the system and the other end of which is connected to ground. These arresters have been successful but have presented problems in that after sparkover the arcs tend to wander over the faces of the metal plates and may reach the edges of the metal plates with the result that the entire arrester can sparkover in a continuous arc extending from one end of the stack of plates to the other. The presence of such an arc greatly reduces the voltage drop across the gaps as compared to the usual voltage drop across the plurality of gaps and may even reduce the voltage drop to a value little more than the value of the voltage drop across a single one or a few of the gaps of the arrester. This makes it more difficult for the arrester to shut off at current zero and makes it more likely that the arrester will restrike as the voltage of the power system increases beyond current zero. For this reason it has been necessary to use discs of comparatively large diameter which results in an increase in costs in the discs and an increase in the cost, size and weight of the enclosure.
Various means have been proposed to prevent the arcs from wandering to the edges of the plates and to prevent the extension of the arcs along the edges of the entire stack. See, for example, U.S. Pat. Nos. 2,888,608, 2,623,192 and 3,660,725. These constructions all add to the cost, bulk and weight of the arresters. It has also been proposed to provide multi-gap arresters embodying ceramic or porcelain plates with electrodes mounted on either side of them and the plates arranged in a stack. Because of the thickness of the plates and the cost of the material such arresters are bulky and more expensive than arresters embodying thin metal plate electrodes.